In some nuclear power facilities and radioactive material handling facilities that are exposed to irradiation of neutrons generated as a result of nuclear fission in nuclear reactors or irradiation of neutrons generated by accelerators, some stable isotopes in materials of neutron generating apparatus, apparatus surrounding them, equipment and structures receive neutron irradiation to be activated and to become radioisotopes. After stopping operation or terminating service of such apparatus, those radioisotopes survive in the apparatus.
It is vitally important to accurately evaluate the quantity of radioisotopes generated as a result of radioactivation by neutron irradiation. That is, it is important from the viewpoint of: planning work projects; taking radiation protection measures for radiation workers; evaluating the radioactivity level and the amount of the radioactive wastes produced as a result of repairing and/or replacing apparatus and equipment and decommissioning facilities; and planning processing and disposal programs.
A radioactivity evaluation method of evaluating radioactivity of an object such as an apparatus, equipment or a structure (to be simply referred to as “object” hereinafter) by sampling parts of the object and using its actually measured values can be used. Such method is disclosed in Japanese Patent Application Laid-Open Publication No. 2005-321249, the entire content of which is incorporated herein by reference.
Radioactivity is generally calculated with the conditions such as the composition of the component elements in the material of the object, the duration of neutron irradiation, the cooling period after neutron irradiation on the object and neutron flux on each energy at each spot. Such method is disclosed in as illustrated in “Basic Procedure to determine the Activity Concentration of Sub-surface Disposal Waste: 2010 (AESJ-SC-F015:2010)”, the Standards Committee of the Atomic Energy Society of Japan.
Neutron energy spectrum changes continuously in sites such as nuclear power plants where objects over a wide region surround a nuclear reactor which is a neutron source. Therefore, a vast amount of calculation is required to accurately evaluate the radioactivity of each spot. That is, the radioactivity of each object needs to be reviewed and recalculated where calculation condition is different from that of calculation completed object. Recalculation is necessary corresponding to the changes of the conditions such as the neutron energy spectrum, the composition of the component elements constituting the material of the object, the nuclide to be evaluated, the duration of neutron radiation, the cooling period or some other condition.
In other words, in order to accurately determine the distribution of radioactivity by radioactivity calculations, it is necessary to perform calculations to reflect the neutron energy spectral distribution in detail. There may arise a need of repeatedly performing a vast amount of radioactivity calculation when the object is a nuclear power plant that covers a wide region and includes the outside of concrete radiation shield surrounding the nuclear reactor or the like.
For planning a processing and disposal program of a facility, it is necessary to evaluate the radioactivity level and the amount of the radioactive wastes produced as a result of decommissioning the facility and so on from early stages. For this purpose, the radioactivity needs to be evaluated in advance.
FIG. 10 is an operational flowchart of a known radioactivity evaluation method.
To begin with, the neutron fluence rate and the neutron energy spectrum on each section in the object is calculated and given. The energy groups of neutron energy spectra are selected by the evaluator. For example, those can be the three energy groups including a fast neutron group, an epithermal neutron group and a thermal neutron group. In the instance of FIG. 10 where there are object sections 1 through J, the radioactivity of those object sections are evaluated sequentially. The radioactivities on a single object section are calculated on all the nuclides k of the section and on each energy group. Radioactivity should be respectively calculated on each point because the result of radioactivity calculations on each point is basically different due to the different neutron energy spectrum and the different material composition on each point.
The number of necessary radioactivity calculation cases increases proportionally as the number of energy groups, the number of the type of nuclides or the number of positions in an object section increases.
A method of dividing the region into some blocks, or sub-regions with a certain size and of performing radioactivity calculations by setting representative neutron energy spectrum condition on each sub-region can be used for the reduction of the radioactivity calculation amount. The neutron energy spectra conditions should be conservative in above method from the point of radiation protection measures and radioactive waste disposal. The method results in overestimation of radiation exposure and radioactive waste amount.
On the other hand, in order to improve the accuracy of evaluation, sub-regions have to be made smaller. Such a subdivision entails a prolonged operation because of an increased amount of radioactivity calculation.
An effective and efficient evaluation method needs to be developed for advance decommissioning planning. While Japanese Patent Application Laid-Open Publication No. 2005-321294 discloses a method of using actually measured values for evaluating the quantity of radioactivity, there is no disclosed technique for efficiently evaluating radioactivity by means of calculations in advance for actual decommissioning planning.